LONDON —The Earth's moon may be a present from Venus, which once had a moon and then lost it, a new theory suggests. Under the theory, Earth's gravity captured Venus' old moon, giving our planet its big natural satellite.

This idea contrasts to the thinking of the vast majority of moon researchers, who believe that the Earth's moon formed some 4.5 billion years ago when a planet-size body slammed into nascent Earth at high speed.

This giant impact hypothesis, however, has its own issues, as did all the alternative moon formation theories discussed this week at the Origin of the Moon conference at the Royal Society here.

"I think part of the key to [understanding] the moon may be that Venus has no moon, and we certainly have to study it (Venus) more," said Dave Stevenson, professor of planetary science at the California Institute of Technology, who proposed the Venus idea at the conference. In an interview with SPACE.com after his presentation, Stevenson said that he himself favored the impact theory on moon formation, but unfortunately this theory did not yet answer all the questions.

The "moon capture" theory assumes that Earth used its gravitational pull to attract a pre-formed space body into its orbit, thus making a satellite of this object.

"The reason why it's interesting is that Earth and Venus are close to each other. They have similar mass, and people think they have probably formed in a similar way," he said. "So the question is, if Earth and Venus formed in similar ways, how come the Earth has a moon and Venus doesn't?"